[Models] Improve iteration over layers (#19497)

Signed-off-by: Lukas Geiger <lukas.geiger94@gmail.com>

vllm/model_executor/models/mpt.py

@@ -4,6 +4,7 @@
 # Adapted from https://huggingface.co/mosaicml/mpt-7b/tree/main
 import math
 from collections.abc import Iterable
+from itertools import islice
 from typing import Optional, Union
 
 import torch
@@ -260,7 +261,7 @@ class MPTModel(nn.Module):
             assert intermediate_tensors is not None
             hidden_states = intermediate_tensors["hidden_states"]
 
-        for block in self.blocks[self.start_layer:self.end_layer]:
+        for block in islice(self.blocks, self.start_layer, self.end_layer):
             hidden_states = block(position_ids, hidden_states)
         if not get_pp_group().is_last_rank:
             return IntermediateTensors({"hidden_states": hidden_states})